The Smithsonian Hectospec Lensing Survey (SHELS) is a magnitude-limited spectroscopically complete survey for R ? 21.0 covering 4 deg2 . SHELS provides a large sample (15,513) of flux-calibrated spectra. The wavelength range covered by the spectra allows empirical determination of k-corrections for the g and r bands from z = 0 to ˜ 0.68 and 0.33, respectively, based on large samples of spectra. We approximate the k-corrections using only two parameters in a standard way: Dn 4000 and redshift, z . We use Dn 4000 rather than the standard observed galaxy color, because Dn 4000 is a redshift-independent tracer of the stellar population of the galaxy. Our approximations for the k-corrections using Dn 4000 are as good as those based on observed galaxy color ( g - r ) (? of the scatter is ˜ 0.08 mag ). The approximations for the k-corrections are available in an online calculator. Our results agree with previously determined analytical approximations from single stellar population (SSP) models fitted to multiband optical and near-infrared photometry for galaxies with a known redshift. Galaxies with the smallest Dn 4000 -the galaxies with the youngest stellar populations-are always attenuated and/or contain contributions from older stellar populations.